The present invention is related to image signal processing and, in particular, to an apparatus and method for decoding compressed image data stored in a file and for displaying the decoded data as undistorted still images.
Previously, the inventors of the present invention have proposed an apparatus and method for generating digital still image files based on digital moving images. Namely, still frames of compressed video data are extracted from digital moving images that had been compressed according to a preselected video signal format. The frames are then stored as data files on a recording medium (such as a hard disk) of a computer. To display the still images on a monitor connected to the computer, the stored image data is retrieved from the files and decoded by processing operations corresponding to the preselected compression format.
It is noted that the image data contained in those files is extracted from the digital moving images that are processed in the following manner. In NTSC systems, for example, the luminance (Y) signal component is sampled at 13.5 MHz to obtain one frame of picture data containing 720 pixels in a horizontal direction by 480 pixels in a vertical direction. In this case, the aspect ratio of the display region occupied by one pixel is 8:9. Hence, in the file generated from the image data of the NTSC system as described above, the aspect ratio of the display region occupied by one pixel is 8:9. The aspect ratio of the picture, represented by an analog picture signal (after conversion from digital domain) and displayed on a typical television set, is 4:3.
In contrast, the aspect ratio of the display region occupied by one pixel on the computer monitor is typically 1:1.
Consequently, when the image file data is decoded and displayed on the computer monitor as a still image of 720 by 480 pixels, the horizontally extended deformed image (visually distorted picture) is displayed. This is the result of the picture being displayed with the 1:1 aspect ratio, while the actual aspect ratio is 8:9.
Similarly, the above problem exists in PAL systems as well. In particular, the number of pixels per one frame of data compressed according to a preselected digital video format is 720 (horizontal) by 576 (vertical); and the aspect ratio of the display region occupied by one pixel is 16:15. As a result, in the PAL system, the picture is displayed with an incorrect aspect ratio on the computer monitor having the 1:1 aspect ratio. That is, the image is visually distorted by appearing elongated in the vertical direction on the display screen.
Further, the digital video signal may be reproduced in the "normal mode" for displaying images with the 4:3 aspect ratio, and also in the "wide mode" for displaying images with the 16:9 aspect ratio. In the wide mode, one frame includes 720 horizontal pixels by 480 vertical pixels in the NTSC system just like in the normal mode, or 720 horizontal pixels by 576 vertical pixels in the PAL system. In both cases, the aspect ratio of the display region occupied by one pixel is horizontally elongated. When the wide mode still image is displayed on the computer monitor (as opposed to the television set), the image appears deformed (shrunk) in the horizontal direction. That is, the picture with the incorrect aspect ratio is obtained.